When an aircraft is in flight, availability of airspeed data is critical and therefore it is necessary to have systems that can be used to measure and/or provide an indication of airspeed of the aircraft. To measure airspeed data that is needed to determine airspeed, many aircraft employ a pitot-static system.
A pitot-static system generally has a pitot tube, a static port, and pitot-static instruments. The pitot-static system is used to obtain pressures for interpretation by the pitot-static instruments. For example, this equipment measures the forces acting on a vehicle as a function of the temperature, density, pressure, and viscosity of the fluid in which it is operating. For instance, an airspeed indicator (ASI) is connected to both the pitot tube and static port. The difference between the pitot pressure and the static pressure is called “impact pressure.” The greater the impact pressure, the higher the indicated airspeed (IAS) is that will be reported.
Other instruments that might be connected can include air data computers, flight control computers, autopilot systems, flight data recorders, altitude recorders, cabin pressurization controllers, and various airspeed switches. For example, many modern aircraft use an air data computer (ADC) to calculate airspeed, rate of climb, altitude, and Mach number. In some aircraft, two ADCs receive total and static pressure from independent pitot tubes and static ports, and the aircraft's flight data computer compares the information from both computers and checks one against the other.
Failure of Pitot-Static Measurement Equipment
Although pitot-static equipment is normally reliable, in some situations pitot-static systems and apparatus can fail. Information obtained from the pitot static system, such as airspeed or altitude, is often critical to a successful and safe flight. As such, errors in pitot-static system readings (or the absence thereof) can be extremely dangerous.
For example, one type of pitot-static system malfunction occurs when a pitot tube is blocked or clogged for some reason, but the static port remains clear. A blocked pitot tube will cause the airspeed indicator to register a faulty or incorrect airspeed. In some cases, this can result in a reading of zero airspeed.
Another type of pitot-static system malfunction occurs when a static port is blocked. A blocked static port is a more serious situation because it affects all pitot-static instruments. One of the most common causes of a blocked static port is airframe icing. A blocked static port will cause the altimeter to freeze at a constant value, the altitude at which the static port became blocked. The vertical speed indicator will freeze at zero and will not change at all, even if vertical airspeed increases or decreases. The airspeed indicator will reverse the error that occurs with a clogged pitot tube and result in an airspeed that is less than it is actually is as the aircraft climbs. When the aircraft is descending, the airspeed will be over-reported. In most aircraft with unpressurized cabins, an alternative static source is available and toggled from within the cockpit of the airplane.
Inherent errors can affect different pitot-static equipment. For example, density errors affect instruments metering airspeed and altitude. This type of error is caused by variations of pressure and temperature in the atmosphere. Therefore, modern pitot-static systems will automatically correct for temperature and pressure variances from standard atmospheric conditions to ensure accurate airspeed data is presented.
Need for Backup Airspeed Measurement Sources
Many modern aircraft implement redundant pitot-static airspeed measurement equipment that can serve as a backup when the primary pitot-static measurement equipment experiences a fault condition or fails. For example, many large transport category aircraft include three very similar or identical pitot-static systems for redundancy.
While the FAA permits this configuration, one drawback of this approach is that the two redundant pitot-static airspeed measurement systems are susceptible to failing for the same reasons that caused the primary pitot-static measurement system to fault or fail. For instance, all three pitot-static measurement systems can fall prey to a common mode failure (e.g., blockage failure due to contamination by ice, volcano ash, bird strikes and/or pitot heater failure, etc.) and experience a fault or failure at the same time. Unfortunately, in such systems, no other backup airspeed measurement system is available.
There is a need for improved backup/redundant systems and apparatus that can be used to provide airspeed measurements during flight of an aircraft in the event that the pitot-static airspeed measurement equipment experiences a fault or fails.
It would be desirable to provide a backup airspeed measurement source for use in emergencies (e.g., when a partial or complete failure of the primary airspeed measurement occurs). It would also be desirable if such backup airspeed measurement sources are not susceptible to the same modes of failure as the primary and secondary pitot-static system(s). Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.